1. Field of the Invention
The present invention relates to a six-cylinder engine capable of changing the number of operating cylinders in accordance with operation conditions.
2. Description of the Related Art
Engines of such a type are disclosed in, for example, Japanese Patent Application Laid-open (kokai) Nos. H08-114133, 2005-90408, 2007-23793, and 2007-162607.
Such an engine (in particular, a V-type or horizontally-opposed-type six-cylinder engine) has problems such as vibration, noise, and difficulty in heat management in a selected cylinder operation state in which some of the cylinders are stopped.
For example, if the positions of cylinders operating in the selected cylinder operation state are not balanced, vibration and/or noise may increase. Further, when specific cylinders are operated continuously for a long period of time or continuously stopped for a long period of time, heat is not generated uniformly within the engine block, so that the uniformity of temperature distribution within the engine block is impaired. Therefore, friction loss may increase due to distortion, or ignitability and/or air-fuel ratio may vary among the cylinders. Moreover, variation of exhaust gas temperature may result in difficulty in satisfactorily performing temperature control for exhaust gas purification catalyst.